1. Field of the Invention
The present invention relates to an intake and exhaust device for a multi-cylinder engine, and more particularly to an intake and exhaust device for a multi-cylinder engine equipped with two intake and exhaust systems each having a turbocharger.
2. Description of the Related Art
In a V-type multi-cylinder engine, turbochargers are disposed corresponding to both banks respectively, and energy of exhaust gas discharged from cylinders in the respective banks is recovered by turbines of the turbochargers to drive a compressor, whereby compressed intake air is supplied to the cylinders. In a V-type multi-cylinder engine equipped with such turbo chargers, as disclosed in, for example, JP 2002-522687 A, exhaust passages of both banks are coupled to each other to balance pressures in both the turbochargers, and a coupling portion between the exhaust passages is further connected to an intake passage common to both the banks by an EGR passage, so that part of exhaust gas is caused to recirculate (EGR) to an intake system. An on/off state of EGR is controlled by opening and closing an EGR valve disposed in the EGR passage.
However, since the EGR valve is disposed in a coupling portion between the exhaust passages for both the banks and the EGR passage, the exhaust passages for both the banks communicate with each other when the EGR valve is open, namely, when EGR is on. In contrast, it is impossible to establish communication between the exhaust passages for both the banks when EGR is off. Thus, in a high-load high-revolution range where EGR is generally off so as to achieve enhancement of output performance, the exhaust passages for both the banks are shut off from each other, and a difference in revolution speed between both the turbochargers, which occurs as a result of a difference in pressure between the exhaust passages for both the banks, is not eliminated. If the difference is small, revolution speeds of both the turbochargers can be made close to their maximums, so that a high output can be generated. However, if the difference is large, the revolution speed of one of the turbochargers may be still lower than its maximum even when the revolution speed of the other turbocharger is close to its maximum. This causes a problem in that it is difficult to sufficiently enhance an output.